Slides is Google's presentation software, similar in function to Microsoft PowerPoint or LibreOffice Impress. You should find most of the functions are replicated and it should be relatively easy to get the hang of.
There's no need! Work in Slides is saved automatically and continuously as you make changes. In the event of a crash, you shouldn't have lost any work at all.
In the upper-right-hand corner there is a button labelled Present. Clicking this will open a new window with your presentation. To make this window full-screen, press the broken square icon on the options menu that is in the lower-left-hand corner, or press F11 on your keyboard. To exit full-screen, click the shrink icon from the options menu or press ESC on your keyboard. To close the presentation, use the X in the upper-right-hand corner of the window, or press ESC on your keyboard.
Slides documents include a full version history by default. You can use this to view any changes you've made to a document, which includes deleting data. From the menu at the top, select File, then Version history and finally See version history. You will be able to see previous versions of the document from the menu on the right-hand-side, along with what was changed and by who. When viewing a previous version, an option to restore it will appear at the top of the page.
Yes! Google Slides supports collaborative editing. When multiple people have the same file open, their identities appear with a colour in the top-right-hand corner, and where they currently are in the presentation will be displayed using the same colour. You can see changes they make as they make them. Changes they make will also be attributed to them on the Version History.